For putting a beat with what the kids are playing in their “improv” game, I wait until they have lost any hesitation about trying something new. Then I show them how I might put a beat with something I might play, using my feet to tap out a straight beat, preferably a rather slow beat so as not to intimidate the youngster. The parent can do the same thing with the youngster at home. When they are at ease with that idea we can set up a metronome beat.
Next step would be to set up a meter as in STRONG, weak, weak, STRONG, weak, weak, or other combinations of strong and weak beats, again preferably slow beats to begin with. I do this first by clapping the beats for the student and getting the parent to join me. As the student gets used to this I can then turn to a metronome which is programmed to give meters. I use simple quarter note beats at first as in 2/4, 3/4 or 4/4 meters. We are still on simple little made up tunes of roughly one phrase. We have not set up phrase parameters other than, start on the key note and finish on the key note. I let all of this percolate for a while.